The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the correction of MRI data acquired during patient motion.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a time varying magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins which may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x, G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The present invention will be described in detail with reference to a variant of the well known Fourier transform (FT) imaging technique, which is frequently referred to as "spin-warp". The spin-warp technique is discussed in an article entitled "Spin-Warp NMR Imaging and Applications to Human Whole-Body Imaging" by W. A. Edelstein et al., Physics in Medicine and Biology, Vol. 25, pp. 751-756 (1980). It employs a variable amplitude phase encoding magnetic field gradient pulse prior to the acquisition of NMR signals to phase encode spatial information in the direction of this gradient. In a two-dimensional implementation (2DFT), for example, spatial information is encoded in one direction by applying a phase encoding gradient (G.sub.y) along that direction, and then a signal is acquired in the presence of a readout magnetic field gradient (G.sub.x) in a direction orthogonal to the phase encoding direction. The readout gradient present during the acquisition encodes spatial information in the orthogonal direction. In a typical 2DFT pulse sequence, the magnitude of the phase encoding gradient pulse G.sub.y is incremented (.DELTA.G.sub.y) in the sequence of "views" that are acquired during the scan to produce a set of NMR data from which an entire image can be reconstructed.
Most NMR scans currently used to produce high resolution 3D medical images, such as the image of coronary arteries, can require a few minutes to acquire the necessary data. Because of the long scan time, patient movement during the scan may be significant and can corrupt the reconstructed image with motion artifacts. There are also many types of patient motion such as respiratory motion, cardiac motion, blood flow, and peristalsis. There are many methods used to reduce or eliminate such motion artifacts including methods for reducing the motion (e.g. breath holding), methods for reducing the effects of motion (e.g. U.S. Pat. No. 4,663,591), and methods for correcting the acquired data for known motion (e.g. U.S. Pat. No. 5,200,700). In the case of respiratory motion, one of the best known methods for reducing motion artifacts is to gate the acquisition of data such that the views are acquired only during a preset portion, or "acquisition window" of the respiratory cycle.
Prior respiratory gating methods employ a means for sensing patient respiration (e.g. U.S. Pat. No. 4,994,473) and producing a gating signal for the MRI system during a preset portion of the respiratory cycle. As long as the gating signal is produced, the MRI system acquires NMR data in the prescribed view order. During other parts of the respiratory cycle the gating signal is turned off and no data is acquired. As a result, when respiratory gating is used the scan time is increased significantly because data can only be acquired over a relatively small portion of each respiratory cycle.
Rather than acquiring NMR data over a relatively short acquisition time, methods are known for acquiring NMR data during subject motion and correcting the data. Such methods often employ a navigator pulse sequence which is interleaved with the acquisition of NMR image data and which is designed to measure subject position. For example, a navigator pulse sequence is disclosed in U.S. Pat. No. 5,363,844 for measuring the position of a patients diaphragm throughout image data acquisition. This position information may be used as described by T. S. Sachs, et al., "Real-Time Motion Detection in Spiral MRI Using Navigators", Magn. Reson. in Med., 32:639-645 (1994) to reject image data acquired during portions of the respiratory or cardiac cycle which produce unacceptable image artifacts. The position information from a navigator echo signal may also be used prospectively as described by M. V. McConnell, "Prospectively Adaptive Navigator Correction for Breath-hold MR Coronary Angiography", Magn. Reson. in Med., 37:148-152 (1997) to adjust the reference phase of the MRI system receiver to correct the subsequently acquired NMR image data. Or, the navigator signal position information may be used to retroactively correct the phase of acquired k-space image data as described by M. E. Brummer, et al., "Reduction Of Respiratory Motion Artifacts In Coronary MRA Using Navigator Echoes", Proc. International Society of Magnetic Resonance in Medicine, 748 (1995).
When acquiring three-dimensional images of moving subjects, such as coronary arteries, it is desirable to acquire the 3D NMR data set from the three-dimensional region of interest by sequentially exciting a series of thin slabs and concatenating the NMR data acquired from the slabs. As described in U.S. Pat. No. 5,167,232 entitled "Magnetic Resonance Angiography By Sequential Multiple Thin Slab Three-Dimensional Acquisition", the thin slabs are overlapped to prevent signal loss at the boundaries due to imperfect slab excitation profiles. When respiratory gating is used, a single respiratory gating window is established and all the data for each slab is acquired during the gating window. The data acquisition for the next slab does not start until the data acquisition for the current slab is completed. This can require a lengthy scan time.